This invention relates to recovery of liquid and gaseous products from subterranean formations containing oil shale, and more particularly, to techniques for providing gas seals in a gas level drift system of an in situ oil shale retort system.
The presence of large deposits of oil shale in the semiarid high plateau Mountain region of the western United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen," which, upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil."
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery liquid and gaseous products from oil shale deposits has been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; 4,043,598; and 4,192,554, which are incorporated herein by this reference. These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded to form a stationary, fragmented permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method for supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the retort and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to product kerogen decomposition, called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbon products, and a residual solid carbonaceous material. The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off gas also can be withdrawn through a gas collection drift system at the bottom of the retort. The products of retorting are referred to herein as liquid and gaseous products.
The off gas may contain nitrogen, hydrogen, carbon monoxide, carbon dioxide, water vapor, methane, and other hydrocarbons, and sulfur compounds such as hydrogen sulfide. Carbon monoxide contained in the off gas is toxic. Hydrogen sulfide is an extremely toxic gas with a toxicity greater than that of hydrogen cyanide. It also possesses a powerful, objectionable odor with a threshold for human smell of about 0.0003 ppm.
The gas collection drift system can be dedicated to collecting off gas from active retorts in the producing region of the formation. That is, the gas collection drifts can be isolated or sealed off to avoid leakage of off gas into adjacent areas where personnel are working, as well as avoiding leakage of air and water into the gas level. Bulkheads can be installed at various locations in the gas collection drift system to keep the off gas sealed within the gas level.
One method of isolating the gas level is to excavate the entire gas collection drift system before retorting is initiated. However, this method is not economical because of the long lead time required in developing the gas level before production can begin. Portions of the gas level drift system can be developed in one region of a tract of retorts while retorting operations are carried on in another region of the retort tract. This is more economical than completing the entire gas level before retorting is started. However, precautions must be taken to ensure that workers in the retort development regions are not exposed to toxic off gas from active retorts in the production region.
It would be desirable to provide an off a gas seal for a gas level drift system that enables a portion of a gas collection drift system to be developed safely, while other mining activities, including retorting, are carried out in an adjacent region of the retort tract. It is also desirable to be able to remove or open such a seal at a later time to connect the gas collection drift system with a newly developed region, preferably from a remote location so that personnel are not exposed to toxic or noxious gases from the gas collection drift system.